The present embodiments relate to an antenna arrangement for magnetic resonance applications.
Known antenna arrangements have a number of antenna elements that run or are substantially parallel to a common central axis and are distributed around the central axis at a distance from the central axis so that the antenna elements enclose a substantially cylindrical volume. The antenna elements in known arrangements are coupled at their ends via end rings and have a number of injection points. When the antenna arrangement is operating in send mode, radio frequency send signals may be injected into the antenna elements, such that corresponding element signals oscillate in the antenna elements. In known arrangements, respective element signals are uniform in respective antenna elements.
Birdcage resonators are constructed using this known arrangement. These known arrangements are also described, for example, in DE 10 2006 011 254 B4, as well as in corresponding US 2009/219024 A1.
Known antenna arrangements are used for sending excitation signals for magnetic resonance signals and for receiving magnetic resonance signals. The signals are, however, received with a relatively low signal-to-noise ratio (SNR). For this reason, prior art magnetic resonance imaging systems often utilize local coils to receive the magnetic resonance signals. Local coils are antenna structures that pick up the radio-frequency signal of the excited spin from the inside of the body of the object under examination. Low-noise pre-amplification may be applied to the radio-frequency signal. After further processing, such as, for example, frequency conversion and/or digitization, the local coils may forward the signal onto the receive and analysis system of the magnetic resonance system. With some known high-field systems (i.e., those having a basic magnetic field of at least 1 Tesla and, in some cases, more than 10 Tesla), the receive antennas are positioned close to the body in order to maximize the SNR in the received signal and, thus the diagnostic information.
The noise of a local coil is primarily caused by three physical effects: (1) the inherent thermal noise of the volume under examination, (2) the thermal noise of the lossy components such as, for example, ohmic losses in metals, dielectric losses in substrates, capacitors, and other discrete components, of the local coil, and (3) the inherent noise of the receive system. The inherent noise of the receive system consists partly of ohmic components and partly of electronic noise (e.g., shot noise, hot electron noise, trap noise, etc.). If the distance between the local coil and the volume under examination (Region of Interest=ROI) is increased to a value greater than approximately 2 cm, the signal received from the ROI falls more quickly than the noise of the local coil caused by the three physical effects described above. In turn, the SNR falls, and, thus, the image quality falls as well.
Using local coils close to the body is, however, disadvantageous. Using local coils in this way results in, for example, a significant additional time penalty, as the local coils must be attached and then removed at a later time. Such a time penalty is not seen when other imaging modalities (e.g., CT) are used. As a result of this time penalty, the efficiency and the throughput of the magnetic resonance system is reduced, which, in turn, increases the cost of the individual examination. In addition, the attachment of local coils adversely affects the patient's comfort and may give rise to claustrophobia. In some cases, this may cause the patient to refuse the examination. Moreover, because the local coils are connected to the receive and analysis system of the magnetic resonance system via screened cables, the magnetic resonance system includes a number of costly and complex connectors, sockets, and/or cables.
Because of these disadvantages, it is desirable to use antennas that are directly connected to the system but are not attached directly to, and are instead remote from, the body. However, because of the great distance between a receiver array (Remote Body Array=RBA) and the body, the receive signal from the ROI is very small. The noise in the receive coils remote from the body is thus dominated by the inherent noise of the coils. To achieve a better SNR, it is thus necessary to reduce the inherent noise of the receive coils. This may be done, for example, by using remote antennas with an extremely low inherent noise. This may be implemented by cooling the remote antennas or by using superconductors (including high-temperature superconductors). The integration of transmit and receive functionality into a common antenna system has already been proposed.
An array remote from the body may be used to take advantage of the benefits of parallel imaging. The array should have the highest possible number of receive antennas, which, in some cases, may be 512 channels. This type of receive array may be embodied as a structure separate from the send antennas. However, the problem with such an arrangement is that the send coil (Body Coil=BC) and the RBA are arranged in the immediate vicinity of one another. Arranging the receive antenna structures in the immediate vicinity of the send arrangement gives rise to couplings and eddy current paths that may, in turn, increase the losses. In addition, an electrical decoupling of the receive antennas from the send antennas becomes significantly more complicated.
Only one or two channels are generally needed to operate the send coil, since the send coil generally only has to be able to generate a circular-polarized excitation signal in the region of interest. In contrast, it is advantageous to construct the receiver arrangement as a multichannel arrangement having significantly more than two channels.